Chapter 2. 2 Literature review

Transcription

1 2 Literature review Chapter 2 This chapter discuss about findings from the literature review. The literature review is focused on layout planning methods, efficiency improvement in layout planning special quality requirements in food processing industry. 2.1 Layout types There are many definitions available for plant layout. According to Riggs, the overall objective of plant layout is to design a physical arrangement that most economically meets the required output, quantity & quality [6]. According to JL Zundi, plant layout ideally involves allocation of space and arrangement of equipment in such a manner that overall operating costs are minimized [7]. In manufacturing systems, three main types of layouts available are product layout, process layout, and group layout. These layouts can be further categorized into flow line, cell, and centre. According to Tompkins [8], the distinction between these types of layout is made based on system characteristics such as production volume and product variety. High Production Volume Product Layout flow line Group Layout cell center Process Layout Low Low Product Variety Figure 2-1: Layout types [8] High 3

2 Product layout (flow shop) is associated with high volume production and low product variety, while process layout (job shop) is associated with low-volume production and high product variety [8]. Figure 2-1, indicates the relationship between production volume and product variety for various layout types Process (functional) oriented layout In this layout similar equipment or functions grouped together, such as all lath machines in one area and all stamping machines in another. A part worked on then travels, according to the established sequence of operations, from area to area. Materials can be routed through the process in any order. This type of layout is typical for hospitals, where areas are dedicated for a particular type of medical care [9]. Traditionally, process oriented layouts have been chosen for manufacturing of a wide range of products in relatively small batches, and a typical production process with a process layout is a job shop [10]. It has the advantages of flexibility, the potential for relatively high utilisation of equipment with lower investment than product layouts and offers high worker satisfaction. However, Evans identifies a number of disadvantages such as high handling and transportation costs, complex planning and control systems, low throughput times and the need for highly skilled operators. Another problem associated with process layouts is the need for high levels of work-in-progress (WIP) to ensure work is always available. This is because it is difficult to pre-specify job priorities [11] Product oriented layout A product oriented layout is defined as a set of interlinked manufacturing resources and cells that simultaneously and in a coordinated manner address the manufacture of a product or a range of similar products, including the necessary assembly work [12]. In this layout, equipment or work processes are arranged according to progressive steps by which the product is made. Production lines for shoes, chemical plants and car washes are all product layouts [9]. Product oriented manufacturing system may be seen as a development of traditional Cellular Manufacturing in the sense that a set of interrelated manufacturing cells may be necessary to completely manufacture a product, or a set of similar products, including assembly. Directing 4

3 systems to the manufacture of specific products can provide competitive advantages that include short production times and improved product quality [12] Cellular layout (group) This is grouping of dissimilar machines into work cells to work on products with similar shapes and processing requirements. A group technology layout is similar to a process layout, in that cells are designed to perform a specific set of processes. It is similar to a product layout, in that cells are dedicated to a limited range of products [9]. The main reason that manufacturing companies are attracted towards implementing cellular manufacturing (CM) layout is that the benefits of CM can normally be realised with relatively low capital investment by relocating and possibly duplicating certain machines as opposed to other automated strategies [13] Fixed position layout The arrangement of machines, storage areas, and/or work areas usually within the confines of a physical structure of a manufacturing facility has significant impacts on shop-floor productivity. Facility layout is often determined by factors such as volume, weight of items to be produced, cost of the building to house the operation, the product mix that must have a facility, and the fragility of the product or component. Although not common, Fixed-Position layouts are normally used when products are too fragile, large, bulky, or heavy to move (e.g., ships and planes). In such configuration, machines, material, and/or workers are moved to an assembly site (often called an assembly island) while products normally remain in one location for its entire manufacturing (assembly) period. Advantages of fixed-position layout include reduced movement of work items: minimized damage or cost of movement and more continuity of the assigned work force since the item does not go from one department to another [14]. 2.2 Layout planning methods Most of the literature of layout design falls into two major categories: algorithmic and procedural approaches. Algorithmic approaches usually simplify both design constraints and objectives in order to reach a surrogated objective function, the 5

4 solution of which can then be obtained. The majority of the existing literature is concentrated on algorithmic approaches [15]. Procedural approaches can be incorporated both qualitative and quantitative objectives in the design process [16] [3] Systematic layout planning In certain type of layout problems, numerical flow of items between departments either being impractical to obtain or does not reveal the qualitative factors that may be crucial to the placement decision. In these situations, the venerable technique known as systematic layout planning (SLP) can be used [9]. As the step 1 SLP begins with a data collection analysis called PQRST for the overall production activities. It includes product (P) quantity (Q) routing (R) supporting (S) and time (T), which should be scrutinized in order to assure the validity of the input data at the design stage. Refer Figure 2-2 for SLP procedure. In step 2 the flow of materials analysis is carried out. All material flows from the whole production line are aggregated into a from-to chart that represents the flow intensity among different tool sets or departments. The step 3 "activity relationships" performs qualitative analysis towards the closeness relationship decision among different departments. In step 4 the relationship diagram locates departments spatially. Those departments that have strong interactions and/or close relationships are placed in proximity. The "space requirements" and "space available" (steps 5 and 6) determine the amount of floor space to be allocated to each department. Space relationship diagram (step 7) adds departmental size information into the relationship diagram of step 4. Additional design constraints and limitations are considered before the start of block layout generation in steps 8 to 9. In step 10, layout alternatives are developed as design candidates. In the final step, the final design is chosen from the design candidates [17] and the alternative layout generation is constituted [3]. 6

5 Figure 2-2: SLP procedure [18] The procedural approach, such as the systematic layout planning procedure has the flexibility to incorporate a variety of design objectives, but is often lacking sound theoretical foundation and credence to be a quality solution [16] [19] Group technology The primary activity in implementing group technology (GT) is grouping parts which require similar processes into families and machines into cells. However, in addition to family and cell formation, other planning and design activities have to be performed when a facility is converted to GT. Paramount among these activities are production planning, process planning, designing the material handling system, determining staffing levels, and developing the layout [20]. Cellular manufacturing (CM) and an application of GT utilises the concept of divide and conquer and involves the grouping of machines, processes and people into cells responsible for 7

6 manufacturing or assembly of similar parts or products [13]. The design for cellular manufacturing involves three stages: (1) Grouping of parts and production equipment into cells (2) Allocation of the machine cells to areas within the shop floor (inter-cell or facility layout) (3) Layout of the machines within each cell (intra-cell or machine layout) [13] Graph theory Graph theoretic approaches also handle the unequal area block plan. In these approaches a block plan is constructed as the dual of a planar graph where nodes represent spaces and links represent required adjacencies [21]. Developing a layout in graph theoretic approach requires the following three steps: (1) Developing an adjacency graph from department relationships (which departments are adjacent) (2) Constructing the dual graph of the adjacency graph (represent departments as adjacent regions having specific boundaries) (3) Converting the dual graph into a block layout (specifying departments with regular shapes and specific areas) [22]. 2.3 Other approaches Other approaches which are also applied to facility layout problem are neural networks, fuzzy logic and expert system [2] Genetic algorithms A genetic algorithm (GA) is a computational method modelled on biological evolutionary process. It can be used to find a nearer-optimal solution to a problem although there may have many near-optimal solutions in the solution terrain. The search process is independent to the problem and the search can be performed under many types of fitness functions [23]. Implementing the GA technique for facility design involves five primary steps: (1) Setting the gene structure (2) Deciding upon the gene evaluation criteria (objective function) 8

7 (3) Generating an initial population of genes (4) Selecting an offspring generation mechanism (5) Coding the process in a computer [24] Since the modelling is done in a computer screen, various sites in the layout have to be indicated as shown in Figure 2-3. The original site shape has to be modified to square shapes for the coding process of computer. Figure 2-3: Site representation [24] To define the position of any facility on the site, a location reference is formulated by using the column and row boundaries of the whole site. The marked cell in Figure 2-3, for example, is located in row 4 and column 4 of the grid of the whole site. The location reference of this cell is calculated as: Location reference = (row position - 1) X total columns + column position Accordingly, the location reference of this cell is (4-1) X = 37. This location reference is used to define the starting position at which a facility is to be placed on the site. The gene structure was set as a string of elements, each corresponding to the location reference of a facility, as shown in Figure 2-4, and the gene length equals 9

8 the total number of facilities. As such, each gene represents one possible solution to the problem. To evaluate the goodness of a possible layout (a gene), an objective function was constructed by multiplying the desired proximity weight between two facilities by the actual distance between them, and summing for all facilities. location reference of a fixed facility location reference of a facility P P+1 P N P Fixed facilities N-P facilities Legend: P = Number of fixed facilities N = Total number of facilities Figure 2-4: Gene formation [24] The objective function, as such, represents the total travel distance associated with a given site layout. Accordingly, minimizing this objective function is required in order to arrive at the optimum layout that results in the least travel distance Non structural fuzzy decision support systems This is a multi objective decision aiding model. This can mainly be used for location of construction site facility layouts. These layouts have to be changed along with the construction process and constrains generated at that point of time. There are three steps in using the model: Decomposition, Comparative judgement and Synthesis of priorities. First decomposition structures a problem into elements of different levels, each independent of those of successive levels, working downwards from goal on the top through criteria bearing to the goal on the second level and then to sub-criteria on the third level, and so on, working from the general (and sometimes uncertain) to the more specific at the lower levels [25]. The merit of using a fuzzy approach is to assign the relative importance of attributes using fuzzy numbers instead of precise numbers [19]. 10

9 Fuzzy-based layout design algorithms modelled the fuzzy or linguistic closeness relationship among departments. The resulting fuzzy scores that represent the desired closeness are then used for a layout design criterion along as part of the layout improvement process. In these methods, the fuzzy closeness determines the order of entry of departments into the layout; but the department placement and departmental dimensions are not explicitly considered [19]. Hence the results obtained are not very realistic in practical implementation. 2.4 Software packages for layout planning These are compute based algorithms designed to support the decision making process Layout optimisation software (LayOPT) LayOPT is a facility layout optimization software package which can be used by layout planners and engineers to solve single and multiple floor facility layout problems. It can be applied to manufacturing, warehouse, office and various service facility layout problems. LayOPT allows the layout planners to generate alternative layout plans quickly and easily and to find the optimal layout among these alternatives. LayOPT is an improvement algorithm that starts with an existing block layout and given the flow and cost data attempts to improve it by exchanging the location of defined departments [26] Automated layout design program (ALDEP) The automated layout design program (ALDEP) starts by selecting the first facility (department) at random and places it starting from a given point that represents top left corner of the site. Then the next facility to be placed is the one that has highest closeness relationship with the first facility. After keeping all facilities on after other, ALDEP uses an objective function to assign a score to the layout and then repeats the process to construct a different layout until user satisfaction is reached [24]. The procedure used to adopt ALDEP is: Step 1: Input following details: Length and width of facility. 11

10 Area of each department. Minimum closeness preference (MCP) value. Sweep width. Relationship chart showing the closeness rating. Location and size of restricted area. Step 2: One department is selected randomly and placed in the layout. Step 3: In this step, the algorithm uses minimum closeness required between departments for the selection of departments to be placed with an earlier placed department. Select the department having maximum closeness rating. If there is no department having minimum closeness preference then any department that remains to be placed is selected. Step 4: If all the departments are placed in the layout, go to Step 5. Else, go to Step 3. Step 5: Compute the total score of the layout. Step 6: If the total score required is the acceptable score, then go to Step 7, else go to Step 2. Step 7: Print the current layout and the corresponding score [27] Computerized relationship layout planning (CORELAP) Computerized relationship layout planning (CORELAP) selects the first facility to be the one with the highest closeness relationship to all others. The next facility to be placed is the one with the highest relationship with the first selected facility. In case of a tie, the facility with the highest relationship all other is selected, and the procedure continues until the layout is completed [24] Computer relative allocation of facilities (CRAFT) CRAFT uses a distance based algorithm [2]. It begins with a user provided layout and uses a more detailed method of calculating the desired closeness relationship between facilities by considering distance, travel cost, and material flow between facilities. It then makes a pair-wise location interchange of facilities that are either 12

11 adjacent or have equal areas, until the layout cost cannot be reduced further [24]. CRAFT input requirements are: 1. Initial layout. 2. Flow data. 3. Cost per unit distance. 4. Total number of departments. 5. Fixed departments and their location. 6. Area of departments. The procedures adopted in CRAFT are. 1. Determine department centroids. Final Layout 2. Calculate rectilinear distance between centroids. 3. Calculate transportation cost for the layout. 4. Consider department sharing a common border. 5. Determine transportation cost of each departmental interchange. 6. Select and implement the departmental interchange that offers the greatest reduction in transportation cost. 7. Repeat the procedure for the new layout until no interchange is able to reduce the transportation cost [27]. These computer algorithms focus on appropriating layout aspects like relationships and material flow. They are not in industry use because of their limitations (unrealistic assumptions, department shapes, consideration of building aspects like utilities and columns, qualitative factors, etc.) [28] Spiral This is a facility layout generation and improvement algorithm. This is an adjacency based algorithm [2]. This is a commercial software available in the market to support the FLP. The algorithm starts with positioning departments from center point and continues like a hologram from center to outside. The aim of any facility layout algorithm is to better allocate the departments within facility. In order to form an initial spiral curve, a block system is used. The width and length of the blocks are given by user and departments are formed according to these values. Then departments are placed around the spiral curve. The initial layout can be selected 13

12 either randomly or with a method which is called enhanced initial layout. Enhanced initial layout finds the highest related department and put it into center. Then adds the other departments according to their relationships with the previous one [22]. 2.5 Efficiency of a layout A well-designed manufacturing layout plan can reduce up to 50% of the operating cost [29] [30]. There are many key performance indicators (KPI) to analyse the efficiency of the layout. These KPI can change with the type of the layout. Main parameters to be considered would be: Distance travelled (m) Distance travelled into weight (mkg) Goods movement People movement Over all equipment efficiency (OEE) Energy efficiency Waste minimization 2.6 Food processing technology The primary concern of food manufacturers is to produce a product that is wholesome, and safe, i.e. free from pathogenic micro organisms, chemical and foreign body contamination [31]. As biological materials, foods deteriorate over time and although this cannot be completely prevented, one aim of food processing is to slow down the rate of deterioration by selecting appropriate methods of processing, ingredient formulations, packaging and storage conditions [32]. Eliminating possibilities of chemical, physical and microbial contamination is a key factor in food processing plant. The layout design should help to minimize the direct and cross contamination. Hence the layout of the food processing factory has to be designed to meet the safety and the efficacy of the product. This is the main difference in the layout design of a food processing plant when compared with other manufacturing plants. Hence common layout design methods cannot be directly applied for food processing plants. 14

13 2.7 Quality standards applicable for food processing Many local and international guide lines are available in the food manufacturing process. Based on management principles, Hazard Analysis Critical Control Point (HACCP) and Good Manufacturing Practice (GMP) have been implemented to help plants to maintain high levels of hygiene [4]. Quality Assurance systems (QA) can be used to improve quality and reduce costs; HACCP programs can be used to better assure food safety. QA and HACCP, implemented in concert, facilitate improvements in product safety and production efficiency [33]. HACCP is widely recognised in the food industry as an effective approach to establishing good production, sanitation, and manufacturing practices that produce safe foods [34]. HACCP systems establish process control through identifying points in the production process that are most critical to monitor and control [35]. A neglected area in food processing is the relation between hygiene and the layout of the processing stages. While food hygiene has been dealt within different disciplines and a considerable amount of knowledge is available, this knowledge is not linked systematically to the evaluation and design of the layout in food processing systems. The literature on layout planning has so far ignored the influence of hygiene factors and the specific nature of food processing companies [4]. 15